1. Field of the Invention
The present invention relates to a photometric apparatus for a camera. More particularly, the present invention relates to a photometric apparatus for a camera which uses a diffractive optical element.
2. Discussion of Related Art
A conventional photometric apparatus for a single-lens reflex camera, for example, is arranged as shown in FIG. 23. A conventional photometric apparatus for a lens shutter camera is arranged as shown in FIG. 24. In these figures, reference numeral 1 denotes a photographic optical system; 2 denotes a finder optical system; 3 denotes a condenser lens for photometry; 4 denotes a light-receiving element for photometry; 5 denotes a quick return mirror; 6 denotes a sub-mirror; 7 denotes a film plane; 8 denotes a photometer window; and 9 denotes a finder window. In many cases, the condenser lens 3 consists essentially of a single lens element by virtue of the spread of aspherical plastic lenses and other similar lenses. This contributes to reductions in the size, thickness and cost of photometric apparatuses.
In recent years, however, there have been increasing demands for reductions in the size, thickness and cost of cameras and for achievement of higher value-added cameras. Under these circumstances, photometric apparatuses must be designed to be capable of meeting these demands.
Recently, the use of diffractive optical elements (hereinafter referred to as "DOE") has begun in consequence of the improvement in optical technology. Examples of prior art that use a DOE in a photometric apparatus for a camera include Japanese Patent Application Unexamined Publication Numbers [hereinafter referred to as "JP(A)"] 5-27304 and 6-250251.
JP(A) 5-27304 proposes a photometric system in which a bundle of light rays is diffracted and reflected by using a hologram, and a part of the ray bundle is separated by a half-mirror and used for photometry.
JP(A) 6-250251 proposes a photometric apparatus for a lens shutter camera in which a DOE is set in front of an aperture stop in a photographic optical system to divide an optical path for photometry.
In particular, lens shutter cameras need to make various improvements in terms of the size of the apparatus, the restriction on design due to the presence of the photometer window, the reliability in the accuracy of photometry, the production cost, etc.
In a case where the condenser lens is formed from a single lens element as has been stated above, the radius of curvature of the condenser lens is reduced in order to ensure an extremely strong power, and the center thickness of the lens is increased in order to ensure the required thickness for the lens edge, causing the apparatus to increase in overall thickness. This has a significant effect on the layout of the interior of the camera and leads to an increase in the overall size of the camera.
The same is said of the presence of the photometer window. In addition, the photometer window restricts the design freedom.
It is conceivable for the optical axis to be bent to make good use of the limited space. However, the provision of a reflecting member or the like leads to a rise in the cost, unfavorably.
Further, parallax may occur because of the external photometry type, and it is difficult to perform photometry according to photographic conditions (e.g. field angle changes during zooming photography or switching between normal and panoramic photography modes). Therefore, it is difficult to obtain high reliability.
In the case of a thick and long lens barrel, as in recent zoom lenses of high zoom ratio, the lens barrel itself may vignette a part of the acceptance angle.
Single-lens reflex cameras are also demanded to be further reduced in size, weight and cost. Some single-lens reflex cameras employ in-finder photometry. In this case, however, the amount of light reaching the film plane and the amount of light reaching the light-receiving element do not coincide with each other. Therefore, the photometry accuracy is unfavorably low.
There are apparatuses that use a Fresnel lens. However, this type of apparatus is not efficient because it suffers from a large loss of light quantity.